Abstract:

Engine exhaust gas feedstream NOx emissions aftertreatment includes a
catalytic device connected upstream of an ammonia-selective catalytic
reduction device including a base metal. Engine operation can be
modulated to generate an engine-out exhaust gas feedstream that converts
to ammonia on the catalytic device. The ammonia is stored on the
ammonia-selective catalytic reduction device, and used to reduce NOx
emissions in the exhaust gas feedstream.

Claims:

1. Method for reducing NOx emissions in an exhaust gas feedstream output
from an internal combustion engine, the method comprising:equipping the
engine with an exhaust aftertreatment system including a catalytic device
fluidly serially connected upstream of an ammonia-selective catalytic
reduction device, the ammonia-selective catalytic reduction device
including catalytic material comprising a base metal;modulating the
engine to generate an engine-out exhaust gas feedstream including nitric
oxide, carbon monoxide, and hydrogen that converts to ammonia on the
catalytic device;storing the ammonia on the ammonia-selective catalytic
reduction device; andreducing NOx emissions in the exhaust gas feedstream
in the ammonia-selective catalytic reduction device using the stored
ammonia.

3. The method of claim 1, wherein modulating the engine to generate the
engine-out exhaust gas feedstream including nitric oxide, carbon
monoxide, and hydrogen is effected when the ammonia-selective catalytic
reduction device is ammonia depleted.

4. The method of claim 3, further comprising discontinuing modulating the
engine to generate nitric oxide, carbon monoxide, and hydrogen in the
exhaust gas feedstream when the ammonia-selective catalytic reduction
device has stored a predetermined amount of ammonia.

5. The method of claim 4, further comprising:equipping the aftertreatment
system with a NOx sensor configured to monitor the exhaust gas feedstream
downstream from the ammonia-selective catalytic reduction device;
andmonitoring a signal output from the NOx sensor to monitor the
ammonia-selective catalytic reduction device for ammonia saturation and
depletion.

6. The method of claim 4, further comprising operating the engine lean of
stoichiometry subsequent to storing ammonia on the ammonia-selective
catalytic reduction device.

7. The method of claim 3, wherein modulating the engine
comprises:operating the engine at one of a stoichiometric air/fuel ratio
and a rich of stoichiometry air/fuel ratio;injecting a first fuel pulse
into a combustion chamber of the engine during each combustion cycle
sufficient to power to engine responsive to an operator torque request;
andinjecting a second fuel pulse into the combustion chamber during a
subsequent stroke of the combustion cycle.

8. The method of claim 1, further comprising operating the engine lean of
stoichiometry subsequent to storing ammonia on the ammonia-selective
catalytic reduction device.

9. The method of claim 8, further comprising:equipping the aftertreatment
system with a NOx sensor configured to monitor the exhaust gas feedstream
downstream from the ammonia-selective catalytic reduction
device;monitoring a signal output from the NOx sensor; anddiscontinuing
operating the engine lean of stoichiometry when the signal output from
the NOx sensor indicates NOx breakthrough downstream from the
ammonia-selective catalytic reduction device.

11. The method of claim 1, wherein modulating the engine comprises
operating the engine within an air/fuel ratio range between 13.5:1 and
14.5:1 to generate the engine-out exhaust gas feedstream comprising
nitric oxide, carbon monoxide, and hydrogen in the exhaust gas
feedstream.

12. Method for reducing NOx emissions in an exhaust gas feedstream from a
spark-ignition, direct-injection internal combustion engine operative
lean of stoichiometry, the method comprising:equipping the engine with an
aftertreatment system including a three-way catalytic converter fluidly
connected upstream of an ammonia-selective catalytic reduction device,
the aftertreatment system including a sensor configured to monitor the
exhaust gas feedstream downstream from the ammonia-selective catalytic
reduction device;operating the engine to generate an exhaust gas
feedstream including nitric oxide, carbon monoxide, and hydrogen that
converts to ammonia on the three-way catalytic converter;storing the
ammonia on the ammonia-selective catalytic reduction device;operating the
engine lean of stoichiometry; andreducing NOx emissions in the
ammonia-selective catalytic reduction device using the stored ammonia.

13. The method of claim 12, further comprising:monitoring signal output of
the sensor configured to monitor the exhaust gas feedstream downstream
from the ammonia-selective catalytic reduction device;wherein operating
the engine lean of stoichiometry is initiated when the signal output of
the sensor indicates the ammonia-selective catalytic reduction device is
ammonia saturated; andwherein operating the engine lean of stoichiometry
is discontinued when the signal output from the sensor indicates NOx
breakthrough downstream from the ammonia-selective catalytic reduction
device.

14. The method of claim 12, further comprising:monitoring a temperature of
the ammonia-selective catalytic reduction device; andcontrolling the
engine at a stoichiometric air/fuel ratio when the temperature of the
ammonia-selective catalytic reduction device is outside a predetermined
temperature range.

19. The exhaust aftertreatment system of claim 16, further comprising a
NOx adsorber device fluidly connected downstream of the ammonia-selective
catalytic reduction device.

20. The exhaust aftertreatment system of claim 16, further comprising a
particulate filter combined with the ammonia-selective catalytic
reduction device including the catalytic material comprising the single
base metal.

Description:

[0001]This application claims the benefit of U.S. Provisional Application
No. 61/049,804 filed on May 2, 2008 which is hereby incorporated herein
by reference.

TECHNICAL FIELD

[0002]This disclosure is related to control of aftertreatment of NOx
emissions in internal combustion engines.

BACKGROUND

[0003]The statements in this section merely provide background information
related to the present disclosure and may not constitute prior art.

[0004]Manufacturers of internal combustion engines are continually
developing new engine control strategies to satisfy customer demands and
meet various regulations. One such engine control strategy comprises
operating an engine at an air/fuel ratio that is lean of stoichiometry to
improve fuel economy and reduce emissions. Such engines include both
compression-ignition and lean-burn spark-ignition engines.

[0005]Lean engine operation may produce oxides of nitrogen (NOx), a known
by-product of combustion, when nitrogen and oxygen molecules present in
engine intake air disassociate in the high temperatures of combustion.
Rates of NOx production follow known relationships to the combustion
process, for example, with higher rates of NOx production being
associated with higher combustion temperatures and longer exposure of air
molecules to the higher temperatures.

[0006]NOx molecules, once produced in the combustion chamber, can be
reduced to nitrogen and oxygen molecules in aftertreatment devices.
Efficacy of known aftertreatment devices are largely dependent upon
operating conditions, such as device operating temperature driven by
exhaust gas flow temperatures and engine air/fuel ratio. Additionally,
aftertreatment devices include materials prone to damage or degradation
in-use due to exposure to high temperatures and contaminants in the
exhaust gas feedstream.

[0007]Known engine operating strategies to manage combustion to increase
fuel efficiency include operating at a lean air/fuel ratio, using
localized or stratified charge combustion within the combustion chamber
while operating in an unthrottled condition. While temperatures in the
combustion chamber can get high enough in pockets of combustion to create
significant quantities of NOx, the overall energy output of the
combustion chamber, in particular, the heat energy expelled from the
engine through the exhaust gas flow can be greatly reduced from normal
values. Such conditions can be challenging to exhaust aftertreatment
strategies, as the aftertreatment devices frequently require elevated
operating temperatures, driven by the exhaust gas flow temperature, to
operate adequately to treat NOx emissions.

[0008]Aftertreatment systems include catalytic devices to generate
chemical reactions to treat exhaust gas constituents. Three-way catalytic
devices (TWC) are utilized particularly in gasoline applications to treat
exhaust gas constituents. Lean NOx adsorbers (NOx trap) utilize catalysts
capable of storing some amount of NOx, and engine control technologies
have been developed to combine these NOx adsorbers with fuel efficient
engine control strategies to improve fuel efficiency and still achieve
acceptable levels of NOx emissions. One known strategy includes using a
lean NOx adsorber to store NOx emissions during lean operations and then
purge and reduce the stored NOx during rich engine operating conditions
with a TWC to nitrogen and water. Particulate filters (DPF) trap soot and
particulate matter in diesel applications, and the trapped material is
periodically purged during high temperature regeneration events.

[0009]One known aftertreatment device comprises a selective catalytic
reduction device (SCR). The SCR device includes catalytic material that
promotes the reaction of NOx with a reductant, such as ammonia or urea,
to produce nitrogen and water. The reductants may be injected into an
exhaust gas feedstream upstream of the SCR device, requiring injection
systems, tanks and control schemes. The tanks may require periodic
refilling and can freeze in cold climates requiring additional heaters
and insulation.

[0010]Catalytic materials used in SCR devices have included vanadium (V)
and tungsten (W) on titanium (Ti) and base metals including iron (Fe) or
copper (Cu) with a zeolite washcoat. Catalytic materials including copper
may perform effectively at lower temperatures but have been shown to have
poor thermal durability at higher temperatures. Catalytic materials
including iron may perform well at higher temperatures but with
decreasing reductant storage efficiency at lower temperatures.

[0011]For mobile applications, SCR devices generally have an operating
temperature range of 150° C. to 600° C. The temperature
range may vary depending on the catalyst. This operating temperature
range can decrease during or after higher load operations. Temperatures
greater than 600° C. may cause reductants to breakthrough and
degrade the SCR catalysts, while the effectiveness of NOx processing
decreases at temperatures lower than 150° C.

SUMMARY

[0012]A method and aftertreatment system for reducing NOx emissions in an
exhaust gas feedstream output from an internal combustion engine includes
a catalytic device connected upstream of an ammonia-selective catalytic
reduction device. The ammonia-selective catalytic reduction device
includes catalytic material comprising a base metal. The engine operation
can be modulated to generate an engine-out exhaust gas feedstream
including nitric oxide, carbon monoxide, and hydrogen that converts to
ammonia on the three-way catalytic device. The ammonia is stored on the
ammonia-selective catalytic reduction device, and used to reduce NOx
emissions in the exhaust gas feedstream during engine operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]One or more embodiments will now be described, by way of example,
with reference to the accompanying drawings in which:

[0014]FIG. 1 is a schematic drawing of an exemplary engine system and
aftertreatment system in accordance with the present disclosure;

[0015]FIG. 2 graphically illustrates exemplary test data from a NOx sensor
and an ammonia sensor as a function of AFR in accordance with the present
disclosure;

[0016]FIG. 3 is a control scheme for managing an exhaust gas feedstream
from the engine in accordance with the present disclosure;

[0017]FIG. 4 graphically depicts exemplary test data describing a
relationship between ammonia production and vehicle speed in accordance
with the present disclosure;

[0022]Referring now to the drawings, wherein the depictions are for the
purpose of illustrating certain exemplary embodiments only and not for
the purpose of limiting the same, FIG. 1 schematically shows an internal
combustion engine 10, aftertreatment system 70, and an accompanying
control module 5 that have been constructed in accordance with an
embodiment of the disclosure. The engine 10 is selectively operative at a
rich air/fuel ratio (AFR), a stoichiometric AFR, and at an AFR that is
primarily lean of stoichiometry. The disclosure can be applied to various
internal combustion engine systems and combustion cycles.

[0023]In one embodiment the aftertreatment system 70 can be connected to
the engine 10 that is coupled to an electro-mechanical hybrid powertrain
system (not shown). The electro-mechanical hybrid powertrain system can
include torque machines configured to transfer tractive power to a
driveline of a vehicle (not shown).

[0024]The exemplary engine 10 comprises a multi-cylinder direct-injection
four-stroke internal combustion engine having reciprocating pistons 14
slidably movable in cylinders 15 which define variable volume combustion
chambers 16. The pistons 14 are connected to a rotating crankshaft 12 by
which linear reciprocating motion is translated to rotational motion. An
air intake system provides intake air to an intake manifold 29 which
directs and distributes air into intake runners of the combustion
chambers 16. The air intake system comprises airflow ductwork and devices
for monitoring and controlling the airflow. The air intake devices
preferably include a mass airflow sensor 32 for monitoring mass airflow
and intake air temperature. A throttle valve 34 preferably comprises an
electronically controlled device that is used to control airflow to the
engine 10 in response to a control signal (ETC) from the control module
5. A pressure sensor 36 in the intake manifold 29 is configured to
monitor manifold absolute pressure and barometric pressure. An external
flow passage recirculates exhaust gases from engine exhaust to the intake
manifold 29, having a flow control valve referred to as an exhaust gas
recirculation (EGR) valve 38. The control module 5 is operative to
control mass flow of exhaust gas to the intake manifold 29 by controlling
opening of the EGR valve 38.

[0025]Airflow from the intake manifold 29 into the combustion chamber 16
is controlled by one or more intake valve(s) 20. Exhaust flow out of the
combustion chamber 16 is controlled by one or more exhaust valve(s) 18 to
an exhaust manifold 39. The engine 10 is equipped with systems to control
and adjust openings and closings of the intake and exhaust valves 20 and
18. In one embodiment, the openings and closings of the intake and
exhaust valves 20 and 18 can be controlled and adjusted by controlling
intake and exhaust variable cam phasing/variable lift control (VCP/VLC)
devices 22 and 24 respectively. The intake and exhaust VCP/VLC devices 22
and 24 are configured to control and operate an intake camshaft 21 and an
exhaust camshaft 23, respectively. The rotations of the intake and
exhaust camshafts 21 and 23 are linked to and indexed to rotation of the
crankshaft 12, thus linking openings and closings of the intake and
exhaust valves 20 and 18 to positions of the crankshaft 12 and the
pistons 14.

[0026]The intake VCP/VLC device 22 preferably includes a mechanism
configured to switch and control valve lift of the intake valve(s) 20 and
variably adjust and control phasing of the intake camshaft 21 for each
cylinder 15 in response to a control signal (INTAKE) from the control
module 5. The exhaust VCP/VLC device 24 preferably comprises a
controllable mechanism operative to variably switch and control valve
lift of the exhaust valve(s) 18 and variably adjust and control phasing
of the exhaust camshaft 23 for each cylinder 15 in response to a control
signal (EXHAUST) from the control module 5.

[0027]The intake and exhaust VCP/VLC devices 22 and 24 each preferably
includes a controllable two-step variable lift control (VLC) mechanism
configured to control magnitude of valve lift, or opening, of the intake
and exhaust valve(s) 20 and 18, respectively, to one of two discrete
steps. The two discrete steps preferably include a low-lift valve open
position (about 4-6 mm in one embodiment) preferably for load speed, low
load operation, and a high-lift valve open position (about 8-13 mm in one
embodiment) preferably for high speed and high load operation. The intake
and exhaust VCP/VLC devices 22 and 24 each preferably includes a variable
cam phasing (VCP) mechanism to control and adjust phasing (i.e., relative
timing) of opening and closing of the intake valve(s) 20 and the exhaust
valve(s) 18 respectively. Adjusting the phasing refers to shifting
opening times of the intake and exhaust valve(s) 20 and 18 relative to
positions of the crankshaft 12 and the piston 14 in the respective
cylinder 15. The VCP mechanisms of the intake and exhaust VCP/VLC devices
22 and 24 each preferably has a range of phasing authority of about
60°-90° of crank rotation, thus permitting the control
module 5 to advance or retard opening and closing of one of intake and
exhaust valve(s) 20 and 18 relative to position of the piston 14 for each
cylinder 15. The range of phasing authority is defined and limited by the
intake and exhaust VCP/VLC devices 22 and 24. The intake and exhaust
VCP/VLC devices 22 and 24 include camshaft position sensors (not shown)
to determine rotational positions of the intake and the exhaust camshafts
21 and 23. The VCP/VLC devices 22 and 24 are actuated using one of
electro-hydraulic, hydraulic, and electric control force, controlled by
the control module 5.

[0028]The engine 10 includes a fuel injection system, comprising a
plurality of high-pressure fuel injectors 28 each configured to directly
inject a mass of fuel into one of the combustion chambers 16 in response
to a signal from the control module 5. The fuel injectors 28 are supplied
pressurized fuel from a fuel distribution system (not shown).

[0029]The engine 10 includes a spark-ignition system (not shown) by which
spark energy can be provided to a spark plug 26 for igniting or assisting
in igniting cylinder charges in each of the combustion chambers 16 in
response to a signal (IGN) from the control module 5.

[0030]The engine 10 is equipped with various sensing devices for
monitoring engine operation, including a crank sensor 42 having output
RPM and operative to monitor crankshaft rotational position, i.e., crank
angle and speed, in one embodiment a combustion sensor 30 configured to
monitor combustion, and an exhaust gas sensor 40 configured to monitor
exhaust gases, comprising an air/fuel ratio sensor in one embodiment. The
combustion sensor 30 comprises a sensor device operative to monitor a
state of a combustion parameter and is depicted as a cylinder pressure
sensor operative to monitor in-cylinder combustion pressure. The output
of the combustion sensor 30 and the crank sensor 42 are monitored by the
control module 5 which determines combustion phasing, i.e., timing of
combustion pressure relative to the crank angle of the crankshaft 12 for
each cylinder 15 for each combustion cycle. The combustion sensor 30 can
also be monitored by the control module 5 to determine a
mean-effective-pressure (IMEP) for each cylinder 15 for each combustion
cycle. Preferably, the engine 10 and control module 5 are mechanized to
monitor and determine states of IMEP for each of the engine cylinders 15
during each cylinder firing event. Alternatively, other sensing systems
can be used to monitor states of other combustion parameters within the
scope of the disclosure, e.g., ion-sense ignition systems, and
non-intrusive cylinder pressure sensors.

[0031]The control module 5 executes algorithmic code stored therein to
control actuators to control engine operation, including throttle
position, spark timing, fuel injection mass and timing, intake and/or
exhaust valve timing and phasing, and exhaust gas recirculation valve
position to control flow of recirculated exhaust gases. Valve timing and
phasing may include negative valve overlap and lift of exhaust valve
reopening (in an exhaust re-breathing strategy). The control module 5 is
configured to receive input signals from an operator (e.g., a throttle
pedal position and a brake pedal position) to determine an operator
torque request and input from the sensors indicating the engine speed and
intake air temperature, and coolant temperature and other ambient
conditions.

[0032]The control module 5 is preferably a general-purpose digital
computer generally comprising a microprocessor or central processing
unit, storage mediums comprising non-volatile memory including read only
memory and electrically programmable read only memory, random access
memory, a high speed clock, analog to digital and digital to analog
circuitry, and input/output circuitry and devices and appropriate signal
conditioning and buffer circuitry. The control module 5 has a set of
control algorithms, comprising resident program instructions and
calibrations stored in the non-volatile memory and executed to provide
the desired functions. The algorithms are preferably executed during
preset loop cycles. Algorithms are executed by the central processing
unit and are operable to monitor inputs from the aforementioned sensing
devices and execute control and diagnostic routines to control operation
of the actuators, using preset calibrations. Loop cycles may be executed
at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100
milliseconds during ongoing engine and vehicle operation. Alternatively,
algorithms may be executed in response to occurrence of an event.

[0033]In operation, the control module 5 monitors inputs from the
aforementioned sensors to determine states of engine operating
parameters. The control module 5 is configured to receive input signals
from an operator (e.g., via a throttle pedal and a brake pedal, not
shown) to determine an operator torque request. The control module 5
monitors the sensors indicating the engine speed and intake air
temperature, and coolant temperature and other ambient conditions.

[0034]The control module 5 executes algorithmic code stored therein to
control the aforementioned actuators to form the cylinder charge,
including controlling throttle position, spark-ignition timing, fuel
injection mass and timing, EGR valve position to control flow of
recirculated exhaust gases, and intake and/or exhaust valve timing and
phasing on engines so equipped. Valve timing and phasing can include
negative valve overlap (NVO) and lift of exhaust valve reopening (in an
exhaust re-breathing strategy) in one embodiment. The control module 5
can operate to turn the engine 10 on and off during ongoing vehicle
operation, and can operate to selectively deactivate a portion of the
combustion chambers 15 or a portion of the intake and exhaust valves 20
and 18 through control of fuel and spark and valve deactivation. The
control module 5 can control AFR based upon feedback from the exhaust gas
sensor 40. The exhaust gas sensor 40 can comprise a wide-range air/fuel
ratio sensor configured to generate a linear signal corresponding to
air/fuel ratio over an air/fuel ratio range. Alternatively, in one
embodiment the exhaust gas sensor 40 can comprise a switch-type
stoichiometric sensor configured to generate an output signal that
corresponds to an air/fuel ratio that is one of rich of stoichiometry and
lean of stoichiometry.

[0035]The exhaust aftertreatment system 70 is fluidly connected to the
exhaust manifold 39 and comprises a catalytic device 48 and an
ammonia-selective catalytic reduction (NH3-SCR) device 50. The catalytic
device 48 is fluidly and serially connected upstream of the
ammonia-selective catalytic reduction device 50. Preferably the catalytic
device 48 is located in an engine compartment and is close-coupled to the
exhaust manifold 39. Preferably the NH3-SCR device 50 is located in an
underfloor location at an extended distance from the catalytic device 48
determined based upon engine and exhaust gas feedstream operating
temperatures and other factors. The exhaust aftertreatment system 70 may
include other catalytic and/or trap substrates operative to oxidize,
adsorb, desorb, reduce, and combust elements of the exhaust gas
feedstream as described herein below.

[0036]The exhaust aftertreatment system 70 can be equipped with various
sensing devices for monitoring the exhaust gas feedstream from the engine
10, including a first NOx sensor 49, a second NOx sensor 52, and an SCR
temperature sensor 51 signally connected to the control module 5. The
first and second NOx sensors 49 and 52 detect and quantify NOx molecules
in the exhaust gas feedstream. The first NOx sensor 49 detects and
quantifies NOx molecules in the exhaust gas feedstream exiting the
catalytic device 48 and entering the NH3-SCR device 50. An additional NOx
sensor 60 may be included in the exhaust aftertreatment system 70 to
detect and quantify NOx molecules in the exhaust gas feedstream entering
the aftertreatment system 70. In one embodiment only the second NOx
sensor 52 is included on the aftertreatment system 70.

[0037]During engine operation, the exemplary engine 10 generates an
exhaust gas feedstream containing constituent elements that can be
transformed in the aftertreatment system, including hydrocarbons (HC),
carbon monoxide (CO), oxides of nitrogen (NOx), and particulate matter
(PM), among others. Oxygen (O2) is present in the exhaust gas
feedstream after operating the engine 10 lean of stoichiometry. Hydrogen
(H2) production can occur in the engine 10 through the combustion
process. Combustion in a stoichiometric or rich AFR environment, wherein
molecular oxygen is scarce, tends to produce elevated levels of molecular
hydrogen.

[0038]The catalytic device 48 performs a number of catalytic functions for
treating an exhaust gas flow. The catalytic device 48 oxidizes
hydrocarbons (HC) and carbon monoxide (CO). The catalytic device 48 is
formulated to produce ammonia during stoichiometric and rich engine
operation. The formulation may involve using varying catalysts including
platinum group metals, e.g., platinum, palladium, and rhodium, with
cerium and zirconium oxides for oxygen storage capacity. In one
embodiment, the catalytic device 48 is a three-way catalytic converter
configured to oxidize hydrocarbons (HC) and carbon monoxide (CO) and
reduce NOx during stoichiometric engine operations.

[0039]The NH3-SCR device 50 reduces NOx into other molecules, including
nitrogen and water as described hereinbelow. An exemplary NH3-SCR device
50 includes a substrate (not shown) coated with a zeolite washcoat and
catalytic material comprising a catalytically active base metal. The
substrate comprises a cordierite or metal monolith with a cell density
about 62 to 93 cells per square centimeter (400-600 cells per square
inch), and a wall thickness about three to seven mils. The cells of the
substrate comprise flow passages through which exhaust gas flows to
contact the catalyst to effect storage of ammonia. The substrate is
impregnated with the zeolite washcoat. The zeolite washcoat also contains
the catalytically active base metals, e.g., iron (Fe), copper (Cu),
cobalt (Co), nickel (Ni). Alternatively, vanadium-based and/or tungsten
(W) on titanium (Ti) compositions may be used as catalysts. Copper
catalysts have been shown to perform effectively at lower temperatures,
e.g., 100° C. to 450° C., but have poor thermal durability.
Iron catalysts may perform well at higher temperatures, e.g., 200°
C. to 650° C., but with decreasing reductant storage capacity.

[0040]The NH3-SCR device 50 stores ammonia to reduce NOx emissions. The
stored ammonia-selectively reacts with NOx in the presence of the
catalytic materials to produce nitrogen and water. The following
equations describe the primary reactions with ammonia within the NH3-SCR
device 50:

4NO+4NH3+O2→4N2+6H2O [1]

3NO2+4NH3→3.5N2+6H2O [2]

2NO+2NO2+4NH3→4N2+6H2O [3]

[0041]Multiple secondary reactions may concurrently occur and will vary
depending on the type of fuel consumed.

[0043]Ammonia may be produced in the catalytic device 48 from a conversion
process described by the following equation:

NO+CO+1.5H2→NH3+CO2 [4]

[0044]One having ordinary skill in the art will appreciate that this
conversion requires molecular oxygen to be depleted from the catalytic
device 48 before NO will react with the molecular hydrogen. In one
embodiment, sufficient conversion occurred at temperatures exceeding
250° C. in the catalytic device 48. Excess oxygen is frequently
present when the internal combustion engine is operated in lean operating
modes, with a lean AFR or with excess air. Thus, the control module 5
controls the AFR to a stoichiometric AFR or rich AFR to deplete oxygen in
the exhaust gas feedstream when ammonia production in the catalytic
device 48 is desired.

[0045]Further, selection of an AFR within the stoichiometric and rich
operating ranges further facilitates ammonia production, for example, by
producing nitric oxide (NO) and hydrogen (H2) in appropriate ratios.
Eq. 4 describes an ideal ratio of 1.5:1 of hydrogen to nitric oxide
(H2:NO). However, based upon the environment provided by the NH3-SCR
device 50 and other reactions taking place within the catalytic device
48, a different actual ratio of hydrogen (H2) to nitric oxide (NO)
can produce ammonia. For example, a ratio of between 3:1 and 5:1 hydrogen
to nitric oxide (H2:NO) is preferred in one embodiment.

[0046]Modulating engine operation includes operating the engine 10 rich or
at stoichiometry while meeting the operator torque request and without
changing engine output power. One exemplary method for operating the
exemplary engine 10 rich of stoichiometry can include executing multiple
fuel injection pulses during a combustion cycle including injecting a
first fuel pulse into the combustion chamber 16 during each compression
stroke. The mass of fuel injected during the first fuel pulse is
determined based upon an amount sufficient to operate the engine 10 to
meet the operator torque request and other load demands. Subsequent fuel
pulses can be injected into the combustion chamber 16 during other
strokes of the combustion cycle thereby generating an exhaust gas
feedstream comprising nitric oxide (NO), carbon monoxide (CO), and
hydrogen (H2) to produce ammonia in the catalytic device 48. In one
embodiment, the subsequent fuel pulses are executed late in a power
stroke or early in an exhaust stroke of the combustion cycle thereby
minimizing likelihood of combustion in the combustion chamber 16.

[0047]Selection of a catalytically active material that enables lower
ratios of hydrogen (H2) molecules to nitric oxide (NO) molecules on
the catalytic device 48 is preferable, as hydrogen requirements directly
relate to an amount of fuel that is consumed by the subsequent fuel
pulses to enable ammonia production. Calibration according to test
results or modeling according to methods sufficient to accurately
estimate engine operation, aftertreatment processes, and conversions can
be utilized to select a preferred AFR to control ammonia production. One
having ordinary skill in the art will appreciate that carbon monoxide
(CO) presence must also be considered to facilitate the reaction
described above.

[0048]Ammonia production can be controlled or enabled according to a
number of factors affecting ammonia usage within the NH3-SCR device 50,
including estimated ammonia storage, estimated or detected ammonia
breakthrough, estimated or detected NOx breakthrough downstream from the
NH3-SCR device 50, and engine operation conducive to ammonia production.
Monitoring of these factors can be accomplished through monitoring a
number of inputs, including engine operation, exhaust gas properties, and
NOx conversion efficiency within the NH3-SCR device 50. For example, the
engine 10 produces higher levels of NOx and hydrogen during engine
acceleration. Such periods conducive to ammonia production can be
utilized to minimize intrusive operation of ammonia production under
engine operating conditions less conducive thereto. Periods of modulating
engine operation to produce ammonia will vary depending upon required
ammonia production, the particulars of the system employed, and the
particular operation of the engine 10.

[0049]FIG. 2 graphically illustrates exemplary test data showing signal
outputs from a known NOx sensor and a known ammonia sensor as a function
of AFR from the engine 10, illustrative of signal outputs from the first
and second NOx sensors 49 and 52 and an ammonia sensor (not shown). Known
NOx sensing technologies do not distinguish between NOx molecules and
ammonia molecules in the exhaust gas feedstream. During lean engine
operating conditions, when ammonia presence in the exhaust gas feedstream
is minimal and NOx molecules are present, signal output from the NOx
sensor indicates NOx molecules and increases with increasing AFR. Signal
output from the ammonia sensor is minimal. At stoichiometric engine
operating conditions, when NOx molecules and ammonia molecules present in
the exhaust gas feedstream are minimal, signal output from the NOx sensor
and the ammonia sensor are minimal. As the AFR decreases during rich
engine operating conditions, the presence of ammonia molecules increase
while NOx molecules are minimal in the exhaust gas feedstream. Signal
output from the NOx sensor and the ammonia sensor increase during rich
engine operation as the AFR decreases. Therefore, during rich engine
operation increased signal output from the first and second NOx sensors
49 and 52 can be used to indicate ammonia molecules in the exhaust gas
feedstream. Thus, ammonia breakthrough may be detected by monitoring
signal output of the second NOx sensor 52 during rich engine operation.
In one embodiment, the second NOx sensor 52 is monitored for increased
signal output during ammonia production. When signal output from the
second NOx sensor 52 increases, the control scheme 200 determines that
ammonia breakthrough is occurring.

[0050]FIG. 3 shows a control scheme 200 for managing an exhaust gas
feedstream from the engine 10 during engine operation. The control scheme
200 is depicted as a plurality of discrete elements. Such illustration is
for ease of description and it should be recognized that the functions
performed by these elements may be combined in one or more devices, e.g.,
implemented in software, hardware, and/or application-specific integrated
circuitry. For example, the control scheme 200 may be executed as one or
more algorithms in the control module 5. The control scheme 200 includes
monitoring the exhaust gas feedstream and the aftertreatment system
(203), including detecting NOx breakthrough and ammonia breakthrough
downstream of the NH3-SCR device 50 using the second NOx sensor 52.
Monitoring the aftertreatment system comprises monitoring temperature of
the NH3-SCR device 50 using the SCR temperature sensor 51.

[0051]Before initiating lean engine operation or modulating engine
operation to produce ammonia, the temperature of the NH3-SCR device 50
must be within a predetermined temperature range (206). In one embodiment
of the NH3-SCR device 50, the predetermined temperature range is
150° C. to 450° C. Preferably, the temperature of the
NH3-SCR device 50 is monitored continuously using the SCR temperature
sensor 51. When the temperature of the NH3-SCR device 50 is outside the
predetermined temperature range, engine operation may be controlled to a
stoichiometric AFR.

[0052]When the temperature of the NH3-SCR device 50 is within the
predetermined temperature range, the control scheme 200 modulates engine
operation to produce nitric oxide (NO), carbon monoxide (CO), and
hydrogen (H2) for ammonia production (209). Ammonia is produced in
the catalytic device 48 as described hereinabove using the nitric oxide
(NO), carbon monoxide (CO), and hydrogen (H2) (212) and transferred
downstream to the NH3-SCR device 50 for storage (215).

[0053]The control scheme 200 can adjust engine operation to discontinue
ammonia production subsequent to determining the NH3-SCR device 50 is
saturated with ammonia (218). Ammonia production can also be discontinued
after a predetermined threshold of ammonia molecules are generated or
when engine operating conditions are not conducive to ammonia production,
e.g., during vehicle decelerations, engine idling, or engine stops.
Ammonia saturation may be estimated based upon a predetermined elapsed
time, or by monitoring the exhaust gas feedstream downstream of the
NH3-SCR device 50 to detect ammonia breakthrough, or determined after
executing a predetermined number of cylinder events. Ammonia breakthrough
may be detected by monitoring signal output of an ammonia sensor (not
shown) configured to monitor the exhaust gas feedstream downstream of the
NH3-SCR device 50. Another method for detecting ammonia breakthrough
comprises monitoring the second NOx sensor 52. During rich engine
operation, signal output from the second NOx sensor 52 increases,
indicate ammonia breakthrough. In one embodiment, saturation may be
estimated using a model according to methods sufficient to accurately
estimate operation of the combustion cycle, aftertreatment processes,
conversions, and monitored operating conditions including intake mass
airflow, AFR, engine speed, TWC temperature, TWC aging state, SCR device
temperature, and SCR device aging state. The model may be calibrated
according to test results corresponding to a particular hardware
application.

[0054]After determining the NH3-SCR device 50 is saturated with ammonia,
the control scheme 200 discontinues modulating engine operation and
ammonia production and transitions engine operation to lean engine
operation (221), resulting in increased NOx emissions into the exhaust
gas flow. The catalytic device 48 reduces a portion of the NOx emissions
transferring oxygen and nitrogen downstream to the NH3-SCR device 50.
Ammonia stored on the catalyst of the NH3-SCR device 50 reacts with NOx
entering the NH3-SCR device 50 thereby reducing NOx emissions and
producing nitrogen and water. The stored ammonia is depleted as ammonia
molecules react with NOx molecules. When the ammonia on the catalyst of
the NH3-SCR device 50 is depleted, NOx emissions pass through the NH3-SCR
device 50 unprocessed.

[0055]Therefore, the control scheme 200 preferably discontinues lean
engine operation after detecting NOx breakthrough downstream from the
NH3-SCR device 50 (224). An increase in signal output from the second NOx
sensor 52 is correlatable to an increase in NOx emissions out of the
NH3-SCR device 50 during lean engine operation, and indicates NOx
breakthrough. Another method for detecting NOx breakthrough comprises
modeling ammonia depletion. Ammonia depletion and therefore NOx
breakthrough may be estimated using a model according to methods
sufficient to accurately estimate operation of the combustion cycle,
aftertreatment processes, conversions, and monitored operating conditions
including intake mass airflow, AFR, engine speed, TWC temperature, TWC
aging state, SCR device temperature, and SCR device aging state. The
model may be calibrated according to test results corresponding to a
particular hardware application. After determining that ammonia is
depleted or detecting NOx breakthrough, the control scheme 200 may
modulate engine operation to produce ammonia (209).

[0056]FIG. 4 graphically depicts exemplary test data describing a
relationship between ammonia production and vehicle speeds. Ammonia
concentrations were measured with a Fourier-transform infrared
spectrometer during engine operations using the exemplary aftertreatment
system 70. As FIG. 4 shows, during engine accelerations, when the
exemplary engine 10 operates at stoichiometry or slightly rich of
stoichiometry (e.g., AFR between 13.8:1 and 14.2:1), ammonia
concentrations produced by the catalytic device 48 can increase.

[0057]FIG. 5 graphically depicts exemplary test data depicting a
relationship between cumulative NOx emissions out of the exemplary engine
10, catalytic device 48, and the NH3-SCR device 50 and vehicle speed.
When the exemplary engine 10 is controlled to alternate between lean and
rich excursions, significantly less NOx emissions pass out of the
aftertreatment system 70 than emitted by the exemplary engine 10 into the
exhaust gas feedstream. FIG. 5 also depicts a NOx reduction by the
NH3-SCR device 50 after NOx reduction in the catalytic device 48.

[0058]The abovementioned methods may be employed in engine systems using
different exhaust aftertreatment configurations, with like numerals
indicating like elements. FIG. 6 shows an engine 10 and an exhaust
aftertreatment system including a particulate filter combined with a TWC
(PF/TWC) 48 upstream of the NH3-SCR device 50. FIG. 7 shows an engine 10
and an exhaust aftertreatment system including a first TWC 48 upstream of
the NH3-SCR device 50 and a second TWC 48' downstream of the NH3-SCR
device 50. The second TWC 48' downstream of the NH3-SCR device 50 may
comprise an oxidation catalyst for managing NH3 breakthrough. FIG. 8
shows an engine 10 and an exhaust aftertreatment configuration including
a TWC 48, a SCR device 50 (NH3-SCR) and a NOx adsorber device 100 (LNT)
downstream of the NH3-SCR device 50. FIG. 9 shows an engine 10 and an
exhaust aftertreatment configuration including a TWC 48, and a NH3-SCR
device combined with a particulate filter 50'.

[0059]FIG. 10 shows another embodiment including an engine 10' and an
exhaust aftertreatment configuration including a close-coupled TWC 48 and
an underfloor converter including a second TWC 48' coupled to a NH3-SCR
device 50. The engine 10' preferably comprises a port-fuel injection
engine that injects fuel into runners of an intake manifold upstream of
each combustion chamber (not shown). The engine 10' is controlled to
operate at or about stoichiometry within a narrowly controlled band for
+/-ΔAFR about stoichiometry which can be an air/fuel ratio band of
14.6:1+/-0.05 in one embodiment.

[0060]FIG. 11 shows a second control scheme 200' comprising a method for
managing an exhaust gas feedstream from the embodiment described with
reference to FIG. 10 comprising the port-fuel injection engine 10' and
the aftertreatment system comprising the TWC 48 and the NH3-SCR device 50
during engine operations, with like elements identified using like
numerals. Although not shown in detail, the embodiment described in FIG.
10 includes an engine-out exhaust gas sensor, a first NOx sensor upstream
of the NH3-SCR device 50, a second NOx sensor downstream of the NH3-SCR
device 50, and a temperature sensor configured to monitor temperature of
the NH3-SCR device 50.

[0061]The control scheme 200' comprises monitoring the exhaust gas
feedstream and the aftertreatment system (203). Monitoring the exhaust
gas feedstream includes detecting NOx breakthrough and ammonia
breakthrough downstream of the NH3-SCR device 50 using the second NOx
sensor. Monitoring the aftertreatment system can include monitoring
temperature of the NH3-SCR device 50 using the SCR temperature sensor.
Before modulating engine operation to produce ammonia, the temperature of
the NH3-SCR device 50 is preferably within a predetermined temperature
range that corresponds to the specific catalytic material comprising a
catalytically active base metal that is used in the NH3-SCR device 50
(206). In one embodiment the predetermined temperature range is
150° C. to 450° C. for the NH3-SCR device 50. Preferably,
the temperature of the NH3-SCR device 50 is monitored continuously using
the SCR temperature sensor. In this control scheme 200', engine operation
is preferably at or near stoichiometry. When the temperature of the
NH3-SCR device 50 is outside the predetermined temperature range, engine
operation is controlled to prevent operation in a fuel cutoff mode, e.g.,
during decelerations, and prevent autonomic engine stops.

[0062]FIG. 12 shows ammonia production (NH3) corresponding to engine-out
air/fuel ratio (A/F Ratio) in the exhaust gas feedstream downstream of a
close-coupled three-way catalytic converter for an exemplary system at
several engine loads (Low, Medium, High) at a predetermined engine
operating speed (1000 rpm). The results indicate that ammonia production
maximizes at an air/fuel ratio of about 14:1, and within an air/fuel
ratio range between 13.5:1 and 14.5:1, thus indicating a preferred
air/fuel ratio point for maximizing ammonia production. The results
further indicate that there is some ammonia production during
stoichiometric operation when the engine-out air/fuel ratio oscillates
rich and lean of stoichiometry, including when the air/fuel ratio
oscillates rich and lean of stoichiometry with an expanded band for
+/-ΔAFR about stoichiometry.

[0063]When the temperature of the NH3-SCR device 50 is within the
predetermined temperature range, the control scheme 200' modulates engine
operation to produce the nitric oxide (NO), carbon monoxide (CO), and
hydrogen (H2) for ammonia production (209'). In this embodiment,
modulating engine operation may include operating the engine 10' at
stoichiometry, operating the engine 10' at stoichiometry with an expanded
band for +/-ΔAFR about stoichiometry, e.g., 14.6:1+/-0.2 in one
embodiment, and operating the engine 10' at or about an air/fuel ratio of
14:1, depending upon an anticipated need for ammonia. The ammonia
produced in the catalytic device 48 as described hereinabove using the
nitric oxide (NO), carbon monoxide (CO), and hydrogen (H2) (212), is
transferred downstream to the NH3-SCR device 50 for storage (215) while
monitoring the NH3-SCR device 50 for saturation (218). So long as the
NH3-SCR device 50 does not saturate, the control scheme 200' may operate
within this loop to manage the exhaust gas feedstream.

[0064]When the control scheme 200' determines the NH3-SCR device 50 is
saturated with ammonia (218), engine operation can be adjusted to
discontinue modulating engine operation to ammonia production (221').
This includes responding to commands for engine operating conditions that
are not conducive to ammonia production, including fuel cutoff events,
e.g., during deceleration events and engine stopping, and transitioning
engine operation to lean engine operation. The control scheme 200'
discontinues modulating engine operation to produce ammonia when the
NH3-SCR 50 saturates, and transitions engine operation to lean engine
operation resulting in increased NOx emissions into the exhaust gas
feedstream. Lean engine operation can include operating at an air/fuel
ratio of about 16.0:1. The stored ammonia is depleted as ammonia
molecules react with NOx molecules. Although not shown explicitly, the
engine 10' may be commanded to operate lean of stoichiometry in response
to engine and vehicle operation, including fuel cutoff events, e.g.,
during deceleration events, engine idling, and engine stopping events, as
can occur with engine stop/start systems associated with hybrid
powertrain system operation. The control scheme 200' preferably
discontinues lean engine operation after detecting NOx breakthrough
downstream from the NH3-SCR device 50 (224). After determining that
ammonia is depleted or detecting NOx breakthrough, the control scheme
200' may modulate engine operation to produce ammonia (209').

[0065]FIG. 13 graphically shows engine-out air/fuel ratio (A/F), vehicle
speed (MPH), and ammonia generation (NH3) over a series of acceleration
and deceleration events for an exemplary vehicle including an engine 10'
and aftertreatment system configured as described with reference to FIG.
10, with the engine 10' operating at stoichiometry. The series of
acceleration and deceleration events are analogous to an FTP-18 driving
cycle. The results indicate a substantial amount of ammonia being
produced during stoichiometric engine operation.

[0066]FIG. 14 graphically shows vehicle speed (MPH), and NOx emissions
into and out of the NH3-SCR device 50 which comprises an Fe-SCR device
over a series of acceleration and deceleration events for an exemplary
vehicle including an exemplary engine 10 and aftertreatment system, with
the engine 10 operating at stoichiometry. The series of acceleration and
deceleration events are analogous to an FTP-18 driving cycle. The results
indicate a reduction in NOx emissions across the NH3-SCR device in the
presence of ammonia produced during stoichiometric engine operation.

[0067]FIG. 15 graphically shows NOx conversion efficiency (%)
corresponding to temperature across an NH3-SCR device using copper as the
catalytic material. The results indicate that there is low conversion
efficiency when there is no oxygen (O2) present, but that with low
levels of oxygen, e.g., 0.5% concentration in the feedstream, the
conversion efficiency increased substantially, including conversion
efficiency in excess of 80% at 0.5% oxygen concentration in the
feedstream when the temperature was at or above 350° C. FIG. 16
graphically shows NOx conversion efficiency (%) corresponding to
temperature across an NH3-SCR device using iron as the catalytic
material. The results indicate that there is low conversion efficiency
when there is no oxygen (O2) present, but that with low levels of
oxygen concentration in the feedstream, e.g., 0.05% or 500 ppm, the
conversion efficiency increased substantially, including a conversion
efficiency in excess of 60% at 0.05% oxygen concentration in the
feedstream when the temperature was at or above 350° C. The
results of FIGS. 15 and 16 indicate that there can be substantial NOx
conversion at exhaust gas feedstream conditions having low levels of
oxygen, e.g., as occurs at stoichiometric engine operation.

[0068]The method described herein contemplates production of ammonia
through engine modulation, utilizing components of the exhaust gas
feedstream to sustain aftertreatment of NOx in an SCR device. It will be
appreciated that these methods can be used in isolation from urea
injection, with the methods described supplying all of the required
ammonia. Alternatively, the methods described herein can be used to
complement a urea injection system, extending the range of the system
between required filling of a urea storage tank while allowing a full
range of engine and powertrain operation without significant monitoring
of ammonia production cycles and ammonia storage capacity, due to
available urea injection on demand.

[0069]The disclosure has described certain preferred embodiments and
modifications thereto. Further modifications and alterations may occur to
others upon reading and understanding the specification. Therefore, it is
intended that the disclosure not be limited to the particular
embodiment(s) disclosed as the best mode contemplated for carrying out
this disclosure, but that the disclosure will include all embodiments
falling within the scope of the appended claims.